supplicant:  Fix crash when cleaning up stations.

This might have been caused by my earlier changes to
the scanning related code.

    type=0x5f5daa "scan", remove_all=0) at wpa_supplicant.c:3382
    at scan.c:1307
    at wpa_supplicant.c:426
    notify=0, terminate=0) at wpa_supplicant.c:3849
    iface=0x199f2d0) at wpa_supplicant.c:3933

Signed-off-by: Ben Greear <greearb@candelatech.com>